Various kinds of combustible materials are contained in a lithium secondary battery. As a result, the lithium secondary battery may be heated or explode due to the overcharge of the lithium secondary battery, the overcurrent in the lithium secondary battery, or other physical external impact applied to the lithium secondary battery. That is, the safety of the lithium secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the lithium secondary battery, such as the overcharge of the lithium secondary battery or the overcurrent in the lithium secondary battery, are disposed on a battery cell in a state in which the safety elements are connected to the battery cell.
Meanwhile, a plurality of battery cells may be encased to manufacture a battery pack using the following three methods.
In a first method, the battery cells are encased in a plastic case. This method is suitable for an external battery pack, which is exposed outward after the battery pack is mounted in a laptop computer or a mobile phone. However, this method has problems in that a thin polymer battery pack exhibits relatively low solidity and, in particular, needle type objects may easily penetrate the battery pack.
In a second method, the battery cells are encased in an insulative tape. This method is suitable for a structure in which a battery pack is mounted in a laptop computer or a mobile phone and is then encased in a plastic case since the battery cells are encased in the insulative tape and thus exhibits low mechanical strength. A battery pack manufactured through this process may be referred to as an inner pack or an embedded pack. However, this method has problems in that it is not suitable for an external battery pack since the insulative tape constitutes a sheathing member.
In a third method, the battery cells are encased in a metal case. This method is suitable for an external battery pack, which is exposed outward after the battery pack is mounted in a laptop computer or a mobile phone. This method is relatively advantageous in that needle type objects cannot easily penetrate the battery pack. However, the metal case increases the weight of the battery pack, and it is relatively difficult to realize products having complex shapes. Also, an upper case and a lower case are generally assembled using bolts with the result that the number of assembly processes is increased.
Therefore, there is a high necessity for a battery pack having a specific structure that is capable of solving the above-mentioned problems. Specifically, it is necessary to provide a battery pack, in which a plastic case is used, having a specific structure to increase mechanical strength, such as looseness, solidity and bending, and to prevent needle type objects from penetrating the battery pack.
Meanwhile, a battery pack mounted in a laptop computer requires high power and large capacity. To this end, a conventional cylindrical battery pack including a plurality of cylindrical battery cells has been used. In recent years, however, the size of a laptop computer has been reduced, and therefore, there is a high necessity for a slim type battery pack.
Therefore, there is a high necessity for a technology that is capable of using pouch-shaped battery cells to manufacture a slim type battery pack, thereby increasing capacity, and, at the same time, providing a reinforcing member between a pack case and the battery cells, thereby increasing mechanical strength of the pack case.